The invention relates to a process for the manufacture of a mixture of starch and polyvinyl alcohol.
It is known that polyvinyl alcohol in different applications such as adhesives, sizing agent for threads, water dissolvable films and paper coating, exercises a positive influence on the functional properties of the prepared products.
The polyvinyl alcohol employed can moreover vary in properties depending on the application. These properties are determined by the degree of hydrolysis and the molecular weight of the product.
In most cases the polyvinyl alcohol needs to be heated sufficiently long in order to bring the product completely into solution.
Furthermore it frequently occurs that polyvinyl alcohol is employed in combination with starch derivatives which are soluble or dispersible in cold water.
This is particularly the case in paper coating compositions in which in practice mixtures of starch and polyvinyl alcohol are applied.
Such mixtures exert, in the presence of optical whiteners, an advantageous influence on the whiteness of the paper. Moreover the starch as a solution or otherwise is added to the size of whiteners. However a polyvinyl dispersion must first be manufactured which is then heated in order to dissolve the polyvinyl alcohol.
The starch can also be added directly in dry form to the size of optical whiteners. In the paper coating sector for that matter it is the trend to change over to superior dry material coatings.
The addition of a modified starch in dry form to a pigment size, with the intention of raising the dry material content of the coating composition, was already described in U.S. Pat. No. 4,425,452. This method offers the advantage that during the drying of the coated paper less moisture needs to be evaporated, which results in a more economical procedure. According to this patent, however no use is made of polyvinyl alcohol.
In the above-mentioned application but also in many other applications it would thus be very advantageous if the polyvinyl alcohol were very quickly soluble in cold water whereby the polyvinyl alcohol could also be added directly in dry form to the pigment slurry. On the one hand the coating installation would be simplified because no solution and heating installation for the starch and the polyvinyl alcohol are necessary. On the other hand, a coating agent with a higher dry material content would be obtained through which during the drying of the coated paper less energy needs to be employed in order to evaporate the superfluous moisture and/or through which the speed of the coater can be increased.
A cold soluble polyvinyl alcohol, whether or not in combination with a starch or a derivative thereof, however was unknown prior to the present invention.